Fluorescence Spectroscopy for Cell Viability Analysis

Scriptie/Masterproef: Master's Thesis


In this thesis, a new approach for obtaining lasing from InGaAs nanoridges apitaxially grown on silicon is investigated. Instead of using the single nanoridge as in previous works, an array of them is treated as a whole system which behaves as a photonic crystal. The advantage of this approach is to obtain lasing modes which propagates perpendicularly to this array of nanoridges, so that electrodes can be placed at their edges limitting the losses. These electrodes can subsequently be employed to pump the laser, reaching the long standing goal of an electrically pumped laser monolithically integrated on silicon.
Several simulation have been performed in order to understand the relation between the structure of this array of nanoridges and the propagating modes. These simulations shoved that the periodicity of thies array sets the upper limit on the frequency of the guided modes, while the shape and dimensions of the single nanoridge can be used to modify their profile and frequency of operation. Adjusting these parameters, a Gaussian like mode has been obtained at telecom wavelengths (around 1550 nm). Moreover, the use of waveguide on top was successfully tested to increase the confinement in the center of this array, resulting in a profile that resembles the fundamental one of a rib waveguide.
With the aim to test this concept, a pre-existing wafer was modified in the clean-room with the objective of obtaining propagating modes at wavelengths within the gain spectrum of the nanoridges (1250-1400 nm). The fixed periodicities of this sample, however, forced significant constraints on the design, and, in order to overcome these limits, higher order modes were used. In this perspective two possible solution were tested: etching the nanoridges to reduce their dimensions and placing a waveguide on top of them. The realization of the first method yielded poor results, which can be attributed to the complexity of the nanoridges in terms of materials and their locations. The second approach resulted easier to realize, but the designed modes operates at the tail of the gain spectrum (~~~1370 - 1400nm).
Finally, the photo luminescence spectra of the fabricated samples and untouched one were measured. From the spectra of the latter, three peaks were recognizable: two produced by e1-hh1 and e1-hh2 transitions in the quantum wells and the other by the band gap of the barriers. for higher pumping powers, lasing peaks are produce by Fabry-PĆ©rot cavities formed by the reflective facets of the single nanoridge. Concerning the fabricated structures, instead, no lasing peaks were visible. In the case of the dry etched nanoridges, the absence of any emission suggested an over-etching of the nanoridges. In the case of the waveguide on top, high losses due to rough surfaces and poor gain might explain the lack of lasing.
Datum prijs2019
Originele taalEnglish

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